1. Field of the Invention
The invention relates to a circuit arrangement for evaluating a video synchronizing signal formed by line synchronizing pulses whose regular sequence is interrupted between two fields by a field synchronizing pulse and in which the time interval between the edges of a field synchronizing pulse and the edges of the subsequent field synchronizing pulse is characteristic of each field.
2. Description of Related Art
Such a circuit arrangement is disclosed in U.S. Pat. No. 4,683,495 (PHD 84.160); it is utilized in, for example, the coding of video signals. Its main object is to recognize in video synchronizing signals, the line synchronizing pulses, the field synchronizing pulses and also the fields and to produce control signals therefrom, with which the coding procedure is controlled.
The prior art arrangement produces two separate signals, namely a field signal and a field synchronizing signal. Using the arrangement, the pulse intervals between what are commonly referred to as pre-equalizing pulses in a video synchronizing signal are measured. The pre-equalizing pulses are located between the last line synchronizing pulse of a field and a picture frequency pulse. The pulse intervals measured are characteristic of each field.
Thus, in the prior art arrangement, the pulse interval sequence before a field synchronizing signal is utilized to recognize the number of a field. When applied to video signals which are supplied by, for example, a video recorder, this has however the disadvantage that the probability of faulty control signals being supplied is very great. The reason therefore is that with these signals, strong disturbances often occur at the end of a field, so that also the pulse interval sequence before the field synchronizing pulse is involved in the procedure on which the mode of operation of the prior art arrangement is based.